Lake Level vs Outflow Final Chapter

The Ins and Outs & Ups and Downs of
Lake Berryessa

By Peter Kilkus

In & Up

Most of us have heard of the rule of thumb that Lake Berryessa rises
1foot for every 1 inch of rain. As with most rules of thumb, this is not
accurate. Lake level rise depends on many variables like the actual lake level
when it rains and how saturated is the ground.

It usually takes about 3 inches of rain on dry hills before there is
any appreciable runoff into the lake. That’s why the 3-week dry period in
January slowed this year’s rise and it took an inch or more to start the runoff
again.

Also the lake is roughly like a V-shaped bowl, so the higher the level
to start, the more rain it takes to make the lake rise even further.

A look at the actual data during the last few years
shows that the average increase in 2009 was 3.4” rise per inch of rain. In 2010
the number was 7.5” rise per inch of rain. In 2011, that figure (seven day
average) went from 3 inches rise per inch of rainfall in early February to 11
inches per inch in early March to a peak of 13 - 16 inches per inch during the
wettest part of late March.

The lake did go up 61 feet in one year in 94-95 but that was due to an
unusually wet season with two significant storms - a January flood event and a
March flood event. A state report said heavy rains began during the second week
of January, 1995 with the Coast Range north of San Francisco and the upper
Sacramento Valley hardest hit. The Russian River jumped from low flow levels to
near record levels (set in February 1986) in just three days. Levels on the
Napa and Eel rivers, although not as high as in 1986, were well above flood
stage.

A series of early March storms dropped significant precipitation. The
March storms produced a new record stage on the Salinas River near Spreckles
and exceeded the 1986 peak on the Napa River by 0.3 feet.

This Old House was doing a Napa restoration and described the weather
on their web site: The Napa Valley House - The renovation of Dennis Duffy's
1906 Victorian farmhouse in the vineyards of northern California.

“The stunning vistas and rich agricultural and cultural history of Napa
Valley proved irresistible to us back in the winter of 1994-95. If only we'd
known about the rain. And then the skies opened. The rain came down 22 days in
a row that February; 52 inches fell in one month; the nearby Napa River leapt
its banks. Contractor Nolan and crew persevered, however, working in the mud to
pour new foundation walls, frame up the new kitchen and make the structure
weather tight. Scene after scene was shot with cast and crew increasingly
sodden. And then, a week before our wrap, the skies turned blue, the sun
reappeared and the mustard bloomed in all its glory.”

*****

From the Napa Valley
Vintners Harvest updates for 1995:

“A year of weather extremes marked the 1995 vintage, which saw winter
floods, spring rains and a June hailstorm. The dramatic weather events got the
growing season off to a late start, and although summer heat pushed grapes to
maturity, harvest was late and yields were down. Moderate Indian Summer
temperatures permitted extended hang time for red varieties, important to the
development of rich flavors and deep color. Overall, vintners characterized the
year as late, light and luscious.”

*****

Out & Down

During an unusually wet 2011 season, conspiracy theory rumors still
circulated. One was that the lake would have risen faster if the Solano
Irrigation District (SID) and the Solano County Water Agency (SCWA) were not
letting out water so fast. Several years ago some people claimed that the dam
managers were letting out water faster than necessary because electricity
prices were so high that they were trying to make a profit from the power
generated by Lake Berryessa.

The actual Lake Berryessa output is controlled by the water allocations
established when the dam was built, as well as certain Bureau of Reclamation
flow specifications. SID cannot exceed specific annual allocations and water is
not simply flushed for power generation exclusively. Power is an added bonus as
water is sent to consumers.

During February and March of 2011, the power plant was shut down
completely for it’s annual maintenance. There was zero power generation for
that period. What can be a little misleading is that if someone looks over the
edge of the dam they will typically see the very visible release spouting out
through the outlet pipe as it was originally designed.

This is not water passing through the powerhouse. The total power
generated also remains relatively constant from year to year...peaking during
the summer months when agriculture demands also peak.

Winter (Oct 15 through mid March...depending on temperatures) is the
time for filling Lake Berryessa - except for a base amount of release required
to maintain stream flows and dam safety. The Bureau of Reclamation
requires a minimum of 45 cubic feet per second (CFS) to be released through
Monticello at all times. This supports the Lower Putah Creek fishery and
keeps Lake Solano “charged”.

There are many variables when it comes to stream flows and runoff from
the watershed. Soil saturation, ground water levels, sun exposure, air
temps, wind speed and direction plus the downstream demands and how soon those
deliveries begin in the season. So how do we measure input and output and
how does it affect lake levels.

Let’s start with some simple arithmetic.

·1 Cubic Foot per Second equals 1.98 Acre-Foot
per day

·1 AF of water equals 1 foot of water on 1
acre

·The required minimum output of 45 CFS equates
to 89.1 AF per day.

·Lake Berryessa has a surface level of about
19,500 acres when full but decreases as it empties.

·At the minimum output it would take 219 days
for the lake to drop 1 foot when full.

Lake Level: Acre-Feet of Storage per Foot of Level Increase

440’ – 445’: 19,500

430’ – 440’: 18,500

400’ – 430’: 17,050

390’ – 400’: 14,820

380’ – 390’: 13,270

350’ – 380’: 11,500

From December 31, 2005 until January 31, 2006 the lake remained above
440 feet. It hit 443.5 feet maximum level on Jan 2 and didn’t get back down to
440 feet until May 19. Its maximum outflow was 5,363 cfs (10,600 AF/day) on
April 14. The average outflow for April, 2005 was 2,000 cfs or 3,960 AF/day.
This would equate to about a foot drop every 5 days. It rained 6.7 inches
during that same April time frame.

The maximum outflow obviously occurs during the summer. The dam was
originally built to supply irrigation water to Solano County. That is still its
main purpose, although it also provides drinking water to about 500,000 people
in Solano County. The Budweiser brewery in Fairfield has a contract that
requires the use of Lake Berryessa water only. Thus the old joke we’ve heard so
many times… No Lake Berryessa water is used in the Napa Valley, but some does
go to the residential communities around the lake.

The majority of Lake Berryessa water is released during May-September –
the growing season. Peak outflow is about 700 cfs or 1,400 AF/day.

Total outflow in 2010 was 202,825 AF. Total outflow from May 1 – Sept. 30 was 156,588 AF. Only about 46,000 AF were released during the winter. To put that in perspective, evaporation in 2010 total was 4.7 feet. From May to September it was 3.06 feet.

A 3.06 foot decrease when the lake is nearly full equates to about 56,600 AF. As incredible as it may seem, more Lake Berryessa water is lost to evaporation during the summer than is actually released from the lake during the winter. Evaporation is measured in inches using actual evaporation measurements. An evaporation pan, 48" in diameter 10" deep, is located at Markley Cove.

Actual 2010 figures show the lake dropped from 423.55’ on 5/1 to 413.38’ on 9/30 - a 10.17’ decrease

At the 423 foot level the lake has a surface area of about 17,050 AF so the loss in AF was 173,399 AF (10.17’ x 17,050 AF/foot). And evaporation accounted for an actual loss of approximately 52,173 AF (3.06’ x 17,050).

At 700 cfs (1,400 AF/day) summer outflow with a surface area of 17,050 AF at the 430 foot level the lake experiences a one-foot drop every 12 days – which matches observation.

Once again the conclusions are that rumors are only rumors and all conspiracy theories about Lake Berryessa water are false. Numbers don’t lie, but do take some thought to unravel.

*****

Some fun facts about water use from the Montana Department of Natural Resources: